spo_surface_ship_reports.readme Richard E. Moritz SHEBA Project Office Drafted 19 July, 1999 Revised 09 November, 1999 This file provides documentation for the SHEBA dataset named "Surface Ship Reports", version 1.0. Description ----------- This data set consists of the surface weather reports prepared at six-hour intervals by the SHEBA Project Office and coded according to the "Ship's Synoptic Code". The variables reported include cloud base height, visibility, cloud fraction, wind direction, wind speed, air temperature, dew point temperature, sea level pressure, pressure tendency, present weather, past weather, amount of low and middle cloud. At six hourly intervals (00, 06, 12, 18 GMT) the SHEBA Project Office weather observers aboard the Canadian Coast Guard icebreaker "Des Groseilliers" recorded the current, 10-minute average values of temperature, dew point temperature, sea level pressure, wind speed and wind direction from the SPO surface Met Tower #1 (or Tower #2 during the intervals when Tower #1 was out of service). The temperature and wind measurements are made at 10 meters above the surface. The observers also made visual estimates of cloud fraction, cloud base height, visibility, and current and past weather from the deck and bridge of the ship. These observations were coded and entered according to the standard ship's synoptic code, which is divided into 13 groups of characters. The coded observations were entered on standard ship report forms, transmitted for distribution on global weather circuits using the ship's TELEX, and copied to an ASCII file on one of the Project Office laptops. These ASCII files are being submitted as part of the SHEBA data set. Instrument Sampling Interval ---------------------------- The ship reports were made at six hour intervals. The temperature, wind and pressure data used are averages for the 10 minutes nearest to the synoptic hour (00, 06, 12, 18 GMT). Parameters ---------- Latitude Longitude Surface air temperature Dew Point temperature Wind Speed Wind Direction Sea Level Pressure Pressure Tendency Cloud Fraction Pressure Tendency Charac Ceiling Height Visibility Present Weather Past Weather Format ------ The format is the standard "Ship's Synoptic Code", augmented with the year and month. Each line of data refers to one ship report. AA MM BBXX CGDX YYGGi 99LLL QKKKK jkhVV Nddff 1sTTT 2rDDD 4PPP 5appp 7wwWZ 8MCEF AA = Year (97 or 98) MM = Month (1-12) BBXX = Indicator for weather report from a sea station CGDX = Radio call sign of the "Des Groseilliers" YY = Day of the month (01-31) GG = Time of observation (nearest whole hour UTC, 00-23) i = Wind indicator (4 indicates wind speed is measured with an anemometer) 99 = Group indicator LLL = Latitude in degrees and tenths (000-900) Q = Quadrant of the globe (7 implies north of equator, west longitude) KKKK = Longitude in degrees and tenths (0000-3600) j = Precipitation data indicator (4 implies precipitation omitted because precipitation amount is not available) k = Weather data indicator (1 implies that weather group 7 is included) h = Height above sea level of the base of lowest cloud seen (code categories 0-9 and /) VV = Visibility (coded categories 90-99) N = Fraction of the sky covered by clouds of all types (eigths; 9 implies sky obscured, / implies cloud cover indiscernible for other reasons) dd = True direction in tens of degrees from which the wind is blowing (00-36; 99 implies variable) ff = Wind speed in knots 1 = Group indicator s = Sign of dry bulb temperature (0 implies positive or zero, 1 implies negative) TTT = Dry bulb temperature (Degrees and tenths Celsius) 2 = Group indicator r = Sign of dew point temperature (0 implies positive or zero, 1 implies negative) DDD = Dew point temperature (Degrees and tenths Celsius)** 4 = Group indicator PPPP = Sea level pressure in millibars and tenths (when slp > 1000 mb the initial "1" is omitted) 5 = Group indicator a = Characteristic of pressure tendency during three hours preceding the observation (coded categories 0-8) ppp = Amount of pressure tendency in the three hours preceding the observation (millibars and tenths) 7 = Group indicator ww = Present weather (coded categories 00 to 99) W = Past weather type - primary (coded categories, 0-9) Z = Past weather type - secondary (coded categories, 0-9) 8 = Group indicator M = Fraction of the sky covered by all low clouds present (or if no low clouds, the fraction covered by medium clouds present) (eigths; 9 implies sky obscured, / implies cloud cover indiscernible for other reasons) E = Types of low cloud (coded categories 0-9 and /) F = Types of medium cloud (coded categories 0-9 and /) H = Types of high cloud (coded categoreis 0-9 and /) **NOTE: In this file the dew points are reported in degrees and tenths Celsius, which differs from the conventional report in whole degrees. Example ------- 97 11 BBXX CGDX 21124 99761 71472 41595 80902 11203 21222 40079 57006 77071 885// 21 November, 1997 at 1200Z. Latitude 76.1 deg North, Longitude 147.2 deg West, Ceiling height code 5 implies 600-1000 meters, Visibility code 95 implies 2-4 km, 8/8ths cloud fraction, wind from 90 degrees true at 02 knots, dry bulb temperature -20.3 C, dew point -22.2 C, pressure 1007.9 millibars, pressure falling steadily by 0.6 mb during the past three hours, slight snow in flakes at the time of observation, snow or rain and snow mixed during the past six hours (primary), also cloud cover more than 1/2 during that time, 8/8ths low cloud, low cloud type is stratocumulus. Measurement Uncertainties ------------------------- During the polar night, it is difficult to see clouds and other visual phenomena, so there are larger uncertainties in cloud fraction, ceiling height, cloud type, visibility and current weather in the dark periods of winter than at other times during the SHEBA year. It is known there is a negative bias of approximately 5% in total cloud fraction estimated by visual observations in the Arctic winter. No correction has been made for this uncertainty and bias in this data set. Suspected Errors ---------------- Note: throughout the year, the reported dew point temperatures are reported on the basis of thermodynamic equilibrium between saturated air and a plane surface of liquid water (not ice). During the summer, 1998 the humidity sensors on the SPO Met Towers reported higher relative humidity than other sensors operating at SHEBA. Post-field Calibration showed that these sensors had positive biases up to +10% relative humidity after the field experiment. Therefore, it is recommended that the dew point temperatures reported here not be used for quantitative research applications. Acknowledgement --------------- Please acknowledge this data set as follows: "The surface ship report data were provided by the SHEBA Project Office, Univ. of Washington."