Dataset comparison at a glance
| ERA5 | ERA5-Land | 20CRv3 |
|---|
| Resolution | 0.25° (~28×21 km at 40°N) | 0.1° (~11×8 km at 40°N) | ~75 km (distributed as 1°×1°) |
| Coverage | Global, land+ocean | Global, land only | Global, land+ocean |
| Temporal resolution | Hourly | Hourly | 3-hourly (+ daily/monthly means) |
| Period of record | 1940–present | 1950–present | 1836–2015 (experimental to 1806); not extended past 2015 |
| Assimilates | Full modern obs network | Downscaled from ERA5 — no independent assimilation | Surface pressure obs only |
| Ensemble | Single deterministic | Single deterministic | 80 members (mean + spread distributed) |
| Best PA use | Modern-era atmospheric predictors, upper-air composites | Fine-grid land/soil/snow fields, modern era | Century-scale composites, pre-1940 context |
1. ERA5 — 0.25°, hourly, 1940–present
Source: gs://gcp-public-data-arco-era5/ar/full_37-1h-0p25deg-chunk-1.zarr-v3 (via the arco-era5-fetch skill), or the Copernicus Climate Data Store (CDS) directly.
Temperature & Skin/Soil Temp
| Abbrev | Full name | Units | Notes |
|---|
| t2m | 2 metre temperature | °C (native K) | Standard air temperature 2m above ground/water — the baseline temp series for most work. |
| d2m | 2 metre dewpoint temperature | °C (native K) | Moisture proxy at 2m; d2m minus t2m gives dewpoint depression. |
| skt | Skin temperature | °C (native K) | Temperature of the actual land/sea/ice surface (not 2m air) — radiative skin temp, spikes hotter/colder than t2m. |
| mx2t | Maximum temperature at 2m since previous post-processing | °C (native K) | Running max since the last archived step (hourly in ARCO/CDS) — accumulate/step-max, not a true daily max unless resampled. |
| mn2t | Minimum temperature at 2m since previous post-processing | °C (native K) | Companion to mx2t; same step-accumulated caveat. |
| stl1 | Soil temperature level 1 (0–7 cm) | °C (native K) | Layer 1 of 4-layer soil temp profile. |
| stl2 | Soil temperature level 2 (7–28 cm) | °C (native K) | — |
| stl3 | Soil temperature level 3 (28–100 cm) | °C (native K) | — |
| stl4 | Soil temperature level 4 (100–289 cm) | °C (native K) | Deepest layer, damps seasonal cycle. |
| tsn | Temperature of snow layer | °C (native K) | Snowpack internal temperature where snow is present. |
Wind
| Abbrev | Full name | Units | Notes |
|---|
| u10 | 10 metre U wind component | m/s | Eastward wind at 10m — standard surface wind component. |
| v10 | 10 metre V wind component | m/s | Northward wind at 10m. |
| u10n | Neutral wind at 10m, U component | m/s | Wind adjusted to neutral atmospheric stability — used for wind-energy/roughness work, not raw obs-comparable wind. |
| v10n | Neutral wind at 10m, V component | m/s | — |
| u100 | 100 metre U wind component | m/s | Eastward wind at 100m — wind-energy hub height. |
| v100 | 100 metre V wind component | m/s | Northward wind at 100m. |
| i10fg | Instantaneous 10m wind gust | m/s | Peak 3-second-equivalent gust at the exact archived timestep. |
| fg10 | 10m wind gust since previous post-processing | m/s | Max gust over the preceding accumulation step (hourly), not instantaneous. |
Pressure, Stability & Boundary Layer
| Abbrev | Full name | Units | Notes |
|---|
| sp | Surface pressure | hPa (native Pa) | Actual station-level pressure (not sea-level reduced) — use with terrain elevation. |
| msl | Mean sea level pressure | hPa (native Pa) | Sea-level-reduced pressure — what you want for synoptic maps/isobars. |
| blh | Boundary layer height | m | Depth of the well-mixed layer — key for pollutant dispersion, convective initiation timing, diurnal mixing. |
| cape | Convective available potential energy | J/kg | Instability metric — higher values favor stronger convection. |
| cin | Convective inhibition | J/kg | Cap strength suppressing convection; large negative values = strong cap. |
| kx | K index | °C | Classic thunderstorm-potential index combining temp/moisture at 850/700/500 hPa. |
| totalx | Total totals index | °C | Another convective-potential index; complements K index. |
| deg0l | Zero-degree level (freezing level) | m | Height of the 0°C isotherm — critical for rain/snow determination, directly useful for your snowfall work. |
Precipitation & Convection
| Abbrev | Full name | Units | Notes |
|---|
| tp | Total precipitation | mm | Large-scale + convective precip combined — the headline precip variable, accumulated since forecast/analysis step start. |
| lsp | Large-scale precipitation | mm | Stratiform/synoptic-scale component of tp. |
| cp | Convective precipitation | mm | Convective component of tp — useful for separating airmass/pulse convection from frontal rain. |
| sf | Snowfall | m of water equivalent | Total (large-scale+convective) snowfall, water-equivalent depth — not the same as snow depth on ground (sd). |
| lsf | Large-scale snowfall | m w.e. | Stratiform component of sf. |
| csf | Convective snowfall | m w.e. | Convective component of sf. |
| ptype | Precipitation type | code (categorical) | Diagnosed phase at the surface: none/rain/snow/freezing rain/wet snow/mixed — directly relevant to rain/snow line work. |
| crr | Convective rain rate | mm/hr (kg m⁻² s⁻¹ native) | Instantaneous convective rain rate at the archived step. |
| lsrr | Large-scale rain rate | mm/hr | Instantaneous stratiform rain rate. |
| mxtpr | Max total precip rate since previous post-processing | mm/hr | Peak precip intensity within the accumulation window — useful for cloudburst/intensity screening. |
Radiation
| Abbrev | Full name | Units | Notes |
|---|
| ssrd | Surface solar (shortwave) radiation downwards | W/m² (MJ/m² if accumulated) | Total downward SW at surface, all-sky — drives snowmelt/evapotranspiration modeling. |
| strd | Surface thermal (longwave) radiation downwards | W/m² | Downward LW — important for nocturnal cooling/radiation-fog and frost forecasting. |
| ssr | Surface net solar radiation | W/m² | Net SW absorbed at surface (down minus reflected). |
| str | Surface net thermal radiation | W/m² | Net LW at surface (usually a cooling term). |
| tisr | TOA incident solar radiation | W/m² | Top-of-atmosphere insolation — useful as a clear-sky reference/normalization. |
| fdir | Total sky direct solar radiation at surface | W/m² | Direct-beam component of ssrd (excludes diffuse). |
| uvb | Downward UV radiation at the surface | W/m² | UV-B specifically. |
Cloud & Column Water
| Abbrev | Full name | Units | Notes |
|---|
| tcc | Total cloud cover | fraction 0–1 | Combined low/mid/high cloud fraction in the column. |
| lcc | Low cloud cover | fraction 0–1 | — |
| mcc | Medium cloud cover | fraction 0–1 | — |
| hcc | High cloud cover | fraction 0–1 | — |
| cbh | Cloud base height | m | Height of lowest cloud base. |
| tclw | Total column cloud liquid water | kg/m² | Vertically integrated liquid water content. |
| tciw | Total column cloud ice water | kg/m² | Vertically integrated ice water content. |
| tcw | Total column water | kg/m² | All water phases (vapor+liquid+ice) integrated through the column. |
| tcwv | Total column water vapour | kg/m² (=mm) | This is precipitable water — the field to pull instead of integrating q by hand. |
Soil & Vegetation
| Abbrev | Full name | Units | Notes |
|---|
| swvl1 | Volumetric soil water layer 1 (0–7 cm) | m³/m³ | Topsoil moisture — most responsive to recent precip. |
| swvl2 | Volumetric soil water layer 2 (7–28 cm) | m³/m³ | — |
| swvl3 | Volumetric soil water layer 3 (28–100 cm) | m³/m³ | — |
| swvl4 | Volumetric soil water layer 4 (100–289 cm) | m³/m³ | Deep-layer moisture, slow to change — drought-relevant. |
| slt | Soil type | categorical (FAO/USDA class) | Static classification, not time-varying. |
| cvh | High vegetation cover | fraction 0–1 | Static-ish canopy fraction (forest-type cover). |
| cvl | Low vegetation cover | fraction 0–1 | Grass/crop-type cover fraction. |
| lai_hv | Leaf area index, high vegetation | m²/m² | Seasonal canopy density for tall vegetation. |
| lai_lv | Leaf area index, low vegetation | m²/m² | Seasonal canopy density for low vegetation. |
| tvh | Type of high vegetation | categorical | Static vegetation class. |
| tvl | Type of low vegetation | categorical | Static vegetation class. |
Snow & Ice
| Abbrev | Full name | Units | Notes |
|---|
| sd | Snow depth | m of water equivalent (native) | Note: ERA5's raw 'sd' is w.e. depth; divide by rsn/1000-density ratio for physical snow depth, or use ERA5-Land's snow fields which are cleaner for land work. |
| rsn | Snow density | kg/m³ | Bulk density of the snowpack. |
| asn | Snow albedo | fraction 0–1 | Reflectivity of snow surface. |
| es | Snow evaporation | m w.e. | Sublimation/evaporation loss from snowpack. |
| smlt | Snowmelt | m w.e. | Melt rate from the snowpack. |
Surface Fluxes & Friction
| Abbrev | Full name | Units | Notes |
|---|
| slhf | Surface latent heat flux | W/m² | Energy used for evaporation/sublimation. |
| sshf | Surface sensible heat flux | W/m² | Direct heat exchange between surface and air. |
| ishf | Instantaneous surface sensible heat flux | W/m² | Instantaneous version of sshf at the archived step. |
| zust | Friction velocity | m/s | Turbulence/roughness-scaled wind stress metric. |
| ie | Instantaneous moisture flux | kg m⁻² s⁻¹ | Instantaneous evaporation flux. |
Runoff & Evaporation
| Abbrev | Full name | Units | Notes |
|---|
| ro | Runoff | m | Surface + sub-surface runoff combined. |
| sro | Surface runoff | m | Surface-only component. |
| ssro | Sub-surface runoff | m | Sub-surface-only component. |
| e | Evaporation | m of water equivalent | Total evaporation (negative = evaporation in ERA5's sign convention on CDS). |
| pev | Potential evaporation | m | Evaporation that would occur given unlimited water availability — useful for drought indices (e.g., feeding PA-CARE/PADI). |
Orography (static)
| Abbrev | Full name | Units | Notes |
|---|
| z | Geopotential | m²/s² (÷9.80665 for elevation in m) | At the surface, this is effectively terrain height × g — the standard way to get ERA5's model orography. |
| sdor | Standard deviation of orography | m | Sub-gridcell terrain roughness — useful for downscaling/PRISM-style terrain predictors. |
| sdfor | Standard deviation of filtered subgrid orography | m | Smoothed variant of sdor. |
| slor | Slope of sub-gridscale orography | dimensionless | Sub-gridcell slope statistic. |
| anor | Angle of sub-gridscale orography | radians | Sub-gridcell orientation of terrain features. |
| isor | Anisotropy of sub-gridscale orography | dimensionless | How directionally elongated the sub-gridcell terrain is (ridges vs. bumps). |
| lsm | Land-sea mask | fraction 0–1 (usually 0 or 1) | Static land fraction — needed since ERA5 assigns coastal cells partial land. |
Lake Model (FLake)
| Abbrev | Full name | Units | Notes |
|---|
| cl | Lake cover | fraction 0–1 | Fraction of gridcell covered by lake — relevant to Erie/Ontario-adjacent PA divisions. |
| dl | Lake depth | m | Static lake depth used by the FLake scheme. |
| lmlt | Lake mix-layer temperature | °C (native K) | Surface mixed-layer lake temperature. |
| lmld | Lake mix-layer depth | m | Depth of the mixed layer. |
| lblt | Lake bottom temperature | °C (native K) | — |
| ltlt | Lake total layer temperature | °C (native K) | Mean temperature of the full water column. |
| lict | Lake ice temperature | °C (native K) | — |
| licd | Lake ice depth | m | Ice thickness where present — relevant to lake-effect onset/cutoff timing. |
| lshf | Lake shape factor | dimensionless | FLake internal shape parameter for thermocline structure. |
Ocean Waves & Vertical-Integral Diagnostics (not typically needed for land/PA work)
| Abbrev | Full name | Units | Notes |
|---|
| swh, mwp, mwd, pp1d, … | Wave height / period / direction family (~35 params) | various | Full sea-state spectrum for the ocean surface — skip for inland Pennsylvania work. |
| p*.162 series | Vertical integral of X flux/divergence (~35 params) | various (kg/m/s, W/m, etc.) | Column-integrated transport diagnostics (moisture flux, energy flux, mass flux, etc.). vimd (vertically integrated moisture divergence) is the one worth knowing individually — useful for atmospheric-river/moisture-transport work. |
Pressure-level variables (all 37 levels: 1000–1 hPa)
Levels (hPa): 1000, 975, 950, 925, 900, 875, 850, 825, 800, 775, 750, 700, 650, 600, 550, 500, 450, 400, 350, 300, 250, 225, 200, 175, 150, 125, 100, 70, 50, 30, 20, 10, 7, 5, 3, 2, 1
| Abbrev | Full name | Units | Notes |
|---|
| t | Temperature | °C (native K) | 3D temperature field. |
| z | Geopotential | m²/s² (÷9.80665 for height in m) | Geopotential height field at each level — use for trough/ridge analysis. |
| u | U component of wind | m/s | Zonal wind at level. |
| v | V component of wind | m/s | Meridional wind at level. |
| w | Vertical velocity (pressure tendency) | Pa/s | Omega — negative = rising motion. |
| q | Specific humidity | kg/kg | Mixing ratio of water vapor to moist air mass. |
| r | Relative humidity | % | Derived field, level-dependent. |
| vo | Vorticity (relative) | s⁻¹ | Cyclonic/anticyclonic spin of the flow. |
| d | Divergence | s⁻¹ | Horizontal mass divergence. |
| cc | Fraction of cloud cover | fraction 0–1 | Cloud fraction at that pressure level. |
| clwc | Specific cloud liquid water content | kg/kg | — |
| ciwc | Specific cloud ice water content | kg/kg | — |
| crwc | Specific rain water content | kg/kg | — |
| cswc | Specific snow water content | kg/kg | — |
| o3 | Ozone mass mixing ratio | kg/kg | Stratospheric-relevant, rarely needed for surface climate work. |
2. ERA5-Land — 0.1°, hourly, 1950–present
Source: CDS dataset reanalysis-era5-land. Same physics lineage as ERA5, run at finer resolution over land only — atmospheric state aloft is not independently reanalyzed here, only the land-surface response.
| Abbrev | Full name | Units | Notes |
|---|
| t2m | 2 metre temperature | °C (native K) | Same definition as ERA5 but on the finer 0.1° land-only grid — generally the better choice for PA terrain work. |
| d2m | 2 metre dewpoint temperature | °C (native K) | — |
| skt | Skin temperature | °C (native K) | — |
| u10 | 10 metre U wind component | m/s | — |
| v10 | 10 metre V wind component | m/s | — |
| sp | Surface pressure | hPa (native Pa) | — |
| tp | Total precipitation | mm | Note: ERA5-Land precip is not independently observed — it's disaggregated from ERA5's coarser precip, so it inherits ERA5's precip biases at finer spatial resolution rather than adding new skill. |
| sf | Snowfall | m w.e. | — |
| e | Evaporation | m w.e. | — |
| pev | Potential evaporation | m | — |
| evabs | Evaporation from bare soil | m w.e. | Partitioned evaporation component unique to ERA5-Land's more detailed land surface scheme. |
| evaow | Evaporation from open water | m w.e. | — |
| evatc | Evaporation from top of canopy | m w.e. | — |
| evavt | Evaporation from vegetation transpiration | m w.e. | — |
| fal | Forecast albedo | fraction 0–1 | — |
| lai_hv | Leaf area index, high vegetation | m²/m² | — |
| lai_lv | Leaf area index, low vegetation | m²/m² | — |
| ro | Runoff | m | — |
| src | Skin reservoir content | m w.e. | Water intercepted on leaves/surface, available for fast re-evaporation. |
| sd | Snow depth | m w.e. | — |
| swe | Snow depth water equivalent | m w.e. | Explicit SWE field, cleaner than deriving it manually. |
| snowc | Snow cover | % (0–100) | Fractional snow-covered area of the gridcell — directly useful for snow-cover-duration climatologies. |
| rsn | Snow density | kg/m³ | — |
| asn | Snow albedo | fraction 0–1 | — |
| es | Snow evaporation | m w.e. | — |
| smlt | Snowmelt | m w.e. | — |
| tsn | Temperature of snow layer | °C (native K) | — |
| stl1 | Soil temperature level 1 | °C (native K) | — |
| stl2 | Soil temperature level 2 | °C (native K) | — |
| stl3 | Soil temperature level 3 | °C (native K) | — |
| stl4 | Soil temperature level 4 | °C (native K) | — |
| swvl1 | Volumetric soil water layer 1 | m³/m³ | — |
| swvl2 | Volumetric soil water layer 2 | m³/m³ | — |
| swvl3 | Volumetric soil water layer 3 | m³/m³ | — |
| swvl4 | Volumetric soil water layer 4 | m³/m³ | — |
| sro | Surface runoff | m | — |
| ssro | Sub-surface runoff | m | — |
| slhf | Surface latent heat flux | W/m² | — |
| sshf | Surface sensible heat flux | W/m² | — |
| ssr | Surface net solar radiation | W/m² | — |
| str | Surface net thermal radiation | W/m² | — |
| ssrd | Surface solar radiation downwards | W/m² | — |
| strd | Surface thermal radiation downwards | W/m² | — |
| lblt | Lake bottom temperature | °C (native K) | Same FLake set as ERA5, retained for lake gridcells. |
| licd | Lake ice depth | m | — |
| lict | Lake ice temperature | °C (native K) | — |
| lmld | Lake mix-layer depth | m | — |
| lmlt | Lake mix-layer temperature | °C (native K) | — |
| lshf | Lake shape factor | dimensionless | — |
| ltlt | Lake total layer temperature | °C (native K) | — |
3. NOAA-CIRES-DOE 20th Century Reanalysis, Version 3 (20CRv3)
It's V3 — no V4 exists as of this writing. V3 superseded 20CRv2/2c (56-member ensembles, shorter records)
and is the actively-referenced current generation. 20CRv3 assimilates only surface pressure observations
(plus prescribed SST/sea ice) via an ensemble Kalman filter — every other field (winds, upper-air temp, humidity, precip)
is the model's dynamically consistent response to fitting pressure, not independently observed. That's what lets it reach
back to 1836/1806, but confidence should scale with ensemble spread, not just the ensemble mean.
Access: NOAA PSL THREDDS/OPeNDAP, NCAR RDA (ds131.3), or NERSC for full 80-member output.
Surface / single-level fields
| Abbrev | Full name | Units | Notes |
|---|
| PRES | Surface pressure | Pa | 20CRv3's actual data-assimilation input variable — surface pressure obs are literally the only thing this reanalysis ingests. |
| PRMSL | Mean sea level pressure | Pa | Sea-level reduced pressure for synoptic map work. |
| AIR.2M | Air/specific humidity companion temp at 2m | K | Paired with SHUM.2M below. |
| TMAX.2M | Maximum temperature at 2m | K | Daily/subdaily max, from the ensemble mean product. |
| TMIN.2M | Minimum temperature at 2m | K | — |
| SHUM.2M | Specific humidity at 2m | kg/kg | — |
| RH.2M | Relative humidity at 2m | % (kg/kg native in some files) | — |
| UGRD.10M | U wind at 10m | m/s | — |
| VGRD.10M | V wind at 10m | m/s | — |
| PRATE | Precipitation rate at surface | kg m⁻² s⁻¹ (mm/s) | Multiply by seconds in the interval to get accumulated precip — this is the one field going all the way back into the 1800s, since it's model-generated (not assimilated) from the pressure-only analysis. |
| PWAT | Precipitable water | kg/m² (mm) | Column-integrated water vapor — the 20CRv3 equivalent of ERA5's tcwv, no need to integrate q by level. |
| DSWRF | Downward shortwave radiation flux | W/m² | — |
| USWRF | Upward shortwave radiation flux | W/m² | Reflected SW — combine with DSWRF for net. |
| ULWRF | Upward longwave radiation flux | W/m² | — |
| LHTFL | Latent heat net flux | W/m² | — |
| SHTFL | Sensible heat net flux | W/m² | — |
| GFLUX | Ground heat flux at surface | W/m² | — |
| HPBL | Height of PBL | m | Boundary layer depth, 20CRv3's analog to ERA5's blh. |
| ICEC | Ice concentration at surface | fraction | Prescribed boundary condition, not independently assimilated over land. |
| ICETK | Ice thickness at surface | m | — |
| PEVPR | Potential evaporation rate at surface | W/m² (energy-equivalent) | Note the unusual energy units rather than a depth rate — convert carefully if comparing to ERA5's pev. |
Pressure-level fields
Distributed on a reduced standard pressure-level set (~24–28 levels, 1000–10 hPa) in most user-facing files — the full 64-level native model grid isn't part of the routine PSL/NCAR distribution.
| Abbrev | Full name | Units | Notes |
|---|
| TMP | Air temperature | K | 3D temperature — available on the standard NCAR/PSL distribution's ~24–28 pressure levels (native model runs on 64 hybrid-sigma levels to ~0.3 hPa, but that full-resolution set isn't in the routine surface-to-user pressure-level files). |
| HGT | Geopotential height | m | Trough/ridge structure through the depth of the atmosphere back to 1836 (experimental) / 1806 (extended) — the main appeal of this dataset for century-scale composites. |
| VVEL | Vertical velocity (pressure tendency) | Pa/s | Omega. |
| RH | Relative humidity | % (kg/kg native) | — |
| SPFH | Specific humidity | kg/kg | — |
| UGRD | U wind component | m/s | — |
| VGRD | V wind component | m/s | — |